The taste system provides sensory information about the chemical composition of the external world. Taste transduction is one of the most sophisticated forms of chemical-triggered sensation in animals. Signaling of taste is found throughout the animal kingdom, from simple metazoans to the most complex of vertebrates. Sensations associated with taste are thought to involve distinct signaling pathways mediated by receptors, i.e., metabotropic or ionotropic receptors. Cells which express taste receptors, when exposed to certain chemical stimuli, elicit taste sensation by depolarizing to generate an action potential, which is believed to trigger the sensation. This event is believed to trigger the release of neurotransmitters at gustatory afferent neuron synapses, thereby initiating signaling along neuronal pathways that mediate taste perception.
As such, taste receptors specifically recognize molecules that elicit specific taste sensation. These molecules are also referred to herein as “tastants.” Many taste receptors belong to the 7-transmembrane receptor superfamily, which are also known as G protein-coupled receptors (GPCRs). Other tastes are believed to be mediated by channel proteins. G protein-coupled receptors control many physiological functions, such as endocrine function, exocrine function, heart rate, lipolysis, carbohydrate metabolism, and transmembrane signaling.
For example, family C of G-protein coupled receptors (GPCRs) from humans comprises eight metabotropic glutamate (mGlu(1-8)) receptors, two heterodimeric gamma-aminobutyric acid(B) (GABA(B)) receptors, a calcium-sensing receptor (CaR), three taste (T1R) receptors, a promiscuous L-alpha-amino acid receptor (GPRC6A), and five orphan receptors. The family C GPCRs are characterized by a large amino-terminal domain, which binds the endogenous orthosteric agonists. Additionally, allosteric modulators which bind to the seven transmembrane domains of the receptors have also been reported.
In general, upon ligand binding to a GPCR, the receptor presumably undergoes a conformational change leading to activation of a G protein. G proteins are comprised of three subunits: a guanyl nucleotide binding α-subunit, a β-subunit, and a γ-subunit. G proteins cycle between two forms, depending on whether GDP or GTP is bound to the α-subunit. When GDP is bound, the G protein exists as a heterotrimer: the Gαβγ complex. When GTP is bound, the α-subunit dissociates from the heterotrimer, leaving a Gβγ complex. When a Gαβγ complex operatively associates with an activated G protein-coupled receptor in a cell membrane, the rate of exchange of GTP for bound GDP is increased and the rate of dissociation of the bound Gα subunit from the Gαβγ complex increases. The free Gα subunit and Gβγ complex are thus capable of transmitting a signal to downstream elements of a variety of signal transduction pathways. These events form the basis for a multiplicity of different cell signaling phenomena, including for example the signaling phenomena that are identified as neurological sensory perceptions such as taste and/or smell.
Mammals are believed to have five basic taste modalities: sweet, bitter, sour, salty, and umami (the taste of monosodium glutamate). Numerous physiological studies in animals have shown that taste receptor cells may selectively respond to different chemical stimuli. In mammals, taste receptor cells are assembled into taste buds that are distributed into different papillae in the tongue epithelium. Circumvallate papillae, found at the very back of the tongue, contain hundreds to thousands of taste buds. By contrast, foliate papillae, localized to the posterior lateral edge of the tongue, contain dozens to hundreds of taste buds. Further, fungiform papillae, located at the front of the tongue, contain only a single or a few taste buds.
Each taste bud, depending on the species, contains 50-150 cells, including precursor cells, support cells, and taste receptor cells. Receptor cells are innervated at their base by afferent nerve endings that transmit information to the taste centers of the cortex through synapses in the brain stem and thalamus. Elucidating the mechanisms of taste cell signaling and information processing is important to understanding the function, regulation, and perception of the sense of taste.
The gustatory system has been selected during evolution to detect nutritive and beneficial compounds as well as harmful or toxic substances. Outside the tongue, expression of Gαgust has also been localized to gastric and pancreatic cells, suggesting that a taste-sensing mechanism may also exist in the gastrointestinal (GI) tract. Expression of taste receptors has also been found in the lining of stomach and intestine, suggesting that taste receptors may play a role in molecular sensing of therapeutic entities and toxins.
Complete or partial sequences of numerous human and other eukaryotic chemosensory receptors are currently known. Within the last several years, a number of groups including the present assignee Senomyx, Inc. have reported the identification and cloning of genes from two GPCR families that are involved in taste modulation and have obtained experimental results related to the understanding of taste biology. These results indicate that bitter, sweet and amino acid taste, also referred as umami taste, are triggered by activation of two types of specific receptors located at the surface of taste receptor cells (TRCs) on the tongue i.e., T2Rs and T1Rs. It is currently believed that at least 26 to 33 genes encode functional receptors (T2Rs) for bitter tasting substances in human and rodent respectively.
By contrast there are only 3 T1Rs, T1R1, T1R2 and T1R3, which are involved in umami and sweet taste. Structurally, the T1R and T2R receptors possess the hallmark of G protein-coupled receptors (GPCRs), i.e., 7 transmembrane domains flanked by small extracellular and intracellular amino- and carboxyl-termini respectively.
T2Rs have been cloned from different mammals including rats, mice and humans. T2Rs comprise a novel family of human and rodent G protein-coupled receptors that are expressed in subsets of taste receptor cells of the tongue and palate epithelia. These taste receptors are organized in clusters in taste cells and are genetically linked to loci that influence bitter taste. The fact that T2Rs modulate bitter taste has been demonstrated in cell-based assays. For example, mT2R-5, hT2R-4 and mT2R-8 have been shown to be activated by bitter molecules in in vitro gustducin assays, providing experimental proof that T2Rs function as bitter taste receptors. See also T2Rs disclosed in U.S. Pat. No. 7,105,650.
T1R family members in general include T1R1, T1R2, and T1R3, e.g., rT1R3, mT1R3, hT1R3, rT1R2, mT1R2, hT1R2, and rT1R1, mT1R1 and hT1R1. It is known that the three T1R gene members T1R1, T1R2 and T1R3 form functional heterodimers that specifically recognize sweeteners and amino acids. It is generally believed that T1R2/T1R3 combination recognizes natural and artificial sweeteners while the T1R1/T1R3 combination recognizes several L-amino acids and monosodium glutamate (MSG), respectively. For example, co-expression of T1R1 and T1R3 in recombinant host cells results in a hetero-oligomeric taste receptor that responds to umami taste stimuli. Umami taste stimuli include by way of example monosodium glutamate and other molecules that elicit a “savory” taste sensation. By contrast, co-expression of T1R2 and T1R3 in recombinant host cells results in a hetero-oligomeric sweet taste receptor that responds to both naturally occurring and artificial sweeteners.
There is a need in the art to develop various ways of identifying compounds or other entities suitable for modifying receptors and their ligands associated with chemosensory or chemosensory related sensation or reaction. In addition, there is a need in the art for compounds or other entities with such characteristics.